5 steps to head off corn rootworm resistance

Steps that Janet Knodel, North Dakota State University Extension entomologist, says you can take to guard against the development of Bt-resistant corn rootworm are:

May 15, 2013

5 steps to head off corn rootworm resistance

Steps that Janet Knodel, North Dakota State University Extension entomologist, says you can take to guard against the development of Bt-resistant corn rootworm are:

Plant your refuge of non-Bt corn. It provides a gene pool source of non-resistant corn rootworms near the field, which can interbreed with any resistant corn rootworms and reduce the probability of Bt-resistant corn rootworms.

Rotate crops and avoid planting continuous corn. Rotating to a non-host crop, such as soybean, will help break the life cycle of western corn rootworm. Only one year out of corn will benefit producers. Western corn rootworm does not lay eggs in soybeans in North Dakota (or Minnesota), or have the extended diapause (adults emerge after two years instead of one year). The extended diapause for the northern corn rootworm has not been confirmed in North Dakota either. A one-year break to soybeans can increase corn yields by 10% and help reduce weed problems, especially volunteer corn.

Control volunteer corn and grassy weed hosts. Any volunteer corn that survives into late June will allow the larvae of the resistance gene pool to survive. If volunteer corn is allowed to silk, adult females will be attracted to the field for egg laying. Be aggressive on controlling volunteer corn and adopt a zero tolerance.

Use a different Bt corn hybrid that expresses a different Cry protein or multiple Cry proteins from the one that may be performing poorly, and avoid continuous use of the same Cry protein for more than three years in a row. In Minnesota, the majority of the corn fields that have had corn rootworm resistance problems since 2009 have been with the Cry3Bb1 protein. The issue gets more complicated because Bt corn hybrids now contain several Cry proteins that are pyramided together. Most of the pyramided Bt corn hybrids, such as SmartStax RIB, perform well when resistant corn rootworms are present; however, these hybrids rely on one protein (Cry34/35Ab1) for effective control.

As corn rootworms are exposed to these multiple traits more frequently and the refuge of non-Bt corn seed is reduced from 20% to 5% in pyramided Bt corn hybrids, we could see a significant magnification in the selection pressure for resistant corn rootworm. Unfortunately, it is impossible to predict when a Bt trait will fail or how fast corn rootworm resistance will develop in North Dakota or elsewhere.

Use a soil insecticide at planting to ensure control of larvae and prevent corn lodging. Soil insecticides do not change the Bt selection pressure on corn rootworm in the field, since insecticides are effective only around the root zone. However, the benefit of using a soil insecticide will depend on whether the Bt trait is still effective and whether resistant corn rootworms are present. Efficacy of soil insecticides is generally only about 70% for control of corn rootworm larvae. Overall, granular soil insecticides at planting have performed better than seed treatments and liquid soil insecticides.